Performance Assessment of Low-Level Waste Disposal Facilities Using Coupled Unsaturated Flow and Reactive Transport Simu
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Performance Assessment of Low-Level Waste Disposal Facilities Using Coupled Unsaturated Flow and Reactive Transport Simulators Diana H. Bacon1, B. Peter McGrail1, Vicky L. Freedman1, Giancarlo Ventura2, Piero Risoluti2, and Kenneth M. Krupka1 1 Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, U.S.A. 2 Italian National Agency for New Technologies, Energy, and the Environment (ENEA), C.R. Casaccia, Via Anguillarese, 301, 00060 Rome, Italy ABSTRACT Recent advances in the development of reactive chemical transport simulators have made it possible to use these tools in performance assessments (PAs) for nuclear waste disposal. Reactive transport codes were used to evaluate the impacts of design modifications on the performance of two shallow subsurface disposal systems for low-level radioactive waste. The first disposal system, located at the Hanford site in Richland, Washington, is for disposal of lowlevel waste glass. Glass waste blocks will be disposed in subsurface trenches, surrounded by backfill material. Using different waste package sizes and layering had a small impact on technetium release rates to the vadose zone. The second disposal system involves a hypothetical repository for low-level waste in Italy. A model of uranium release from a grout waste form was developed using the STORM reactive transport code. Uranium is predicted to be relatively insoluble for several hundred years under the high-pH environment of the cement pore water. The effect of using different filler materials between the waste packages on uranium flux to the vadose zone proved to have a negligible impact on release rates. INTRODUCTION Immobilized Low Activity Waste (ILAW) PA The Hanford Site in southeastern Washington State has been used extensively to produce nuclear materials for the U.S. strategic defense arsenal by the U.S. Department of Energy (DOE). A large inventory of radioactive and mixed waste has accumulated in 177 buried single- and double-shell tanks. Liquid waste recovered from the tanks will be pretreated to separate the lowactivity fraction from the high-level and transuranic wastes. The low-activity waste (LAW) will be immobilized in glass and placed in a near-surface disposal system on the Hanford Site. Vitrifying the LAW will generate over 160,000 m3 of glass. The immobilized low-activity waste (ILAW) at Hanford is among the largest volumes of waste within the DOE complex and is one of the largest inventories of long-lived radionuclides planned for disposal in a low-level waste facility [1,2]. PA of Italian Low-Level Waste (LLW) Disposal Site Following a referendum decision in November 1987, all 4 of Italy's existing reactors were shut down, and orders for up to 16 new reactors cancelled. A Task Force (known as SITO) was JJ11.42.1
established within ENEA (Ente per le Nuove tecnologie, l'Energia e l'Ambiente, or Agency for New Technologies, Energy and the Environment) in 1996. The Task Force was given the objective of identifying a possible disposal site for low-level waste (LLW) and of designin
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